Move the linear order to the HGraph.
Bug found by Zheng Xu: SsaLivenessAnalysis being a stack allocated
object, we should not refer to it in later phases of the compiler.
Specifically, the code generator was using the linear order, which
was stored in the liveness analysis object.
Change-Id: I574641f522b7b86fc43f3914166108efc72edb3b